The National Electrical Code defines three classes of circuits and provides specific installation requirements for each. In general, Class 2 is defined as any circuit that provides 30V or less at 100 VA. Class 3 is defined as any circuit that provide up to 150V at 100 VA. Class 1 may then be used to classify circuits that provide output that is not power limited.
Lighting systems typically consist of permanently wired-in lighting fixtures, with each lighting fixture obtaining its power directly from a regular Class 1 power line. As a regular power line is not power limited and is considered large enough to be a fire-hazard, the National Electrical Code classifies traditional lighting systems as Class 1 circuits and thus requires numerous protective measures. For example, traditional lighting systems are required by the National Electrical Code to have electrical conductors that are installed in the form of armored cable or within steel conduits.
FIGS. 1 and 2 depict two traditional lighting systems. FIG. 1 shows a traditional lighting system having six troffers 102 connected in parallel to each other. Each troffer typically includes a ballast (not shown) and is connected to a junction box 104 by a whip 106. Each junction box 104 is then connected to an ordinary power outlet 108 via conduit wire that is housed within a steel conduit 110.
FIG. 2 shows another traditional lighting system having three recessed light fixtures 202 connected in series. In this traditional lighting system, each recessed light fixture 202 includes a junction box 204 associated with each fixture and connected to the fixture via a whip 206. As in the system of FIG. 1, each junction box is then connected to an ordinary power outlet via conduit wire that is housed within a steel cable.
These and other traditional lighting systems have numerous drawbacks. First, by delivering a line voltage to each fixture, traditional lighting systems provide a shock hazard and thus present a significant danger during installations. In addition, components such as steel conduit and whips, which are required in Class 1 systems for safety measures, are both costly and inflexible. For example, installation of steel conduit around obstructions can be time-consuming, and any last-minute reconfigurations may become very cumbersome.
A solution to many of the problems associated with traditional lighting systems was introduced by Ole Nilssen in U.S. Pat. No. 4,626,747. Specifically, the Nilssen patent disclosed a lighting system capable of complying with Class 3 power requirements. The Class 3 lighting system includes a power supply capable of being connected to an ordinary power line and converting the non-power limited, low-frequency power line voltage to a power-limited, high-frequency voltage. The Class 3 lighting system also includes a light fixture capable of being connected to the power supply in a location that may be remote from the power supply. Because of the Class-3 output characteristics of the power supply units, the amount of available power in the Nilssen patent was limited to a level considered acceptably safe from a fire initiation viewpoint, yet adequate in power to provide ample light from a fluorescent lighting fixture (e.g. 100 Watts).
By using a Class 3 power supply that mounts remotely from the fixture, the lighting system disclosed in the Nilssen patent eliminates the need for steel conduits, whips and other associated components necessary for traditional Class 1 lighting systems, reduces material expenses and management/inventory costs, and virtually eliminates trade conflict callbacks. Due to the high-frequency operation of the Nilssen Class 3 system, the lamp transformer within each fixture could be small and light weight. Combining this miniaturized transformer with the reduced fixture/structural requirements, due to the Class-3 characteristics, permits the lighting fixtures to be particularly compact and light-of-weight. Furthermore, because of their Class-3 nature, the fixtures in the Nilssen lighting system may also be considered as ordinary portable (plug-in) lighting products; which implies that they may be installed, moved, removed, and/or exchanged by unskilled persons.
However, the Nilssen lighting system did not provide a complete answer. Specifically, the power supply output disclosed in the Nilssen patent was controlled by providing a constant voltage while limiting the output current. This approach results in a significant voltage drop along transmission cables causing the output of a lamp to vary significantly depending on its distance from the power supply. In addition, fixtures designed to operate with the constant voltage power supply disclosed in the Nilssen patent require matching tank circuits, which may increase both the cost and complexity of the lamp driver circuits.